This invention relates in general to a digital projection system and in particular to a closed loop three color alignment system for digital projectors.
In a digital projection system involving the use or three separate light-modulating devices such as LCDs or micro-mirrors, image convergence depends upon the accurate registration of the three separate images created by the light modulating elements. For simplicity sake, we will look at this invention with regard to, but not limited in scope to implementation with an LCD device. With poor image convergence, the contrast and sharpness of the image both suffer, and in addition fringing of the image may occur. For example, a white dress may have a green fringe on one side and a red fringe on the other when shown against a dark background. These degradations are obviously undesirable in a projection system. Adding to the chance of poor convergence is the likelihood of higher and higher resolutions for a variety of applications including digital cinema. With these higher resolutions comes increasing difficulty in achieving and maintaining convergence (usually a fraction of a pixel) due to the smaller pixels on the devices. In a device with large temperature changes such as a projector, even with attention to the thermal expansion of the LCD fixturing and other optical components, positional fixturing without complex designs or extravagant materials has a finite practical limit that is currently the same order of magnitude as the pixel size.
A current method of performing a six-degree of freedom alignment (x, y, z, and rotations around x, y, and z) is to use a fixture to align the image modulating devices and hold them in place while they are fixed using solder or adhesive to a set of pads built into the optical system. The image modulating devices can then be removed from the fixture and re-used. This type of alignment can take a long time especially if the fixture adjustments are not entirely orthogonal and independent. (Rotation adjustments are seldom possible around the exact center of an image modulating device.) For example, in rotating the image modulating device around the z-axis to eliminate a skew error, one could potentially also be altering its position in x and y. This method requires high precision fixturing, and possibly many iterations to achieve the required alignment. Even if the proper alignment is achieved with the fixture, errors in the final position of the image modulating devices can result. For example, though this method is performed with the image modulating devices and projector running, due to the need for access to the device, the system may not have all of it""s covers and cooling operational. Therefore, as the alignment is not performed at the projector""s working temperature, this can lead to positional errors, and a resulting lack of color convergence when the projector is run completely assembled at a different operating temperature. Convergence using this method can also suffer from positional inaccuracies resulting from induced stresses in the joints caused by the solder cooling or the adhesive shrinking. This is a one-time adjustment and does not allow for realignment at a later date.
An alternate method is to provide manual adjustments on some or all of the axes in the projector to enable in-projector alignment. This method is potentially expensive as the precision adjustment remains with the projector, potentially cumbersome in terms of getting adjustments for all of the stages to fit, and difficult to design athermally. For some applications, it may also be possible to rely on simply butting the image modulating devices to datum structures within the projector. Even with precisely ground components and tight manufacturing tolerances, this method is seldom accurate enough for high-resolution systems.
Macauley et al. discloses an image registration system in U.S. Pat. No. 4,683,467 which registers multiple images on a screen. This system makes use of sensors mounted to the screen as input for the correction system. Because the detection takes place at the screen, photodetectors are required to be located on or near the screen detracting from the viewing experience.
Ledebuhr in U.S. Pat. No. 5,170,250 shows an internal image registration system which is suitable for use in a projector. The light valves produce alignment beams which are sensed internal to the projector by photodetectors to control the CRT light valves. A disadvantage of this invention is that a spatial relationship needs to be maintained between the three photodetectors. Such a system is not capable of correcting for either skew or focus. In addition, the correction methods used won""t work for image modulating devices such as LCDs and micro-mirrors which cannot be aligned electronically to high resolution.
Hara et al. in U.S. Pat. No. 5,592,239 disclose a projector device with the capability to properly register the image in translation and rotation. However, the device is only able to be used during an off-line alignment of the projector because the detection system is not suited to closed loop operation whilst projecting images.
There is a need for a method and apparatus for creating and maintaining proper pixel alignment without requiring high-precision fixtures and projector components. Also there is a need for a method and apparatus for performing a continuous image registration in a projection system.
The problems discussed above can be overcome and better performance achieved with a closed loop system for aligning and maintaining the alignment of these devices. Briefly, according to one aspect of the present invention a closed loop three color alignment system for a digital projector comprises a light source and an optical engine which splits a beam of light from the light source into first, second, and third wavelength bands. A first, second, and third spatial light modulator imparts image data and a first, second, and third fiducial data to the first, second, and third wavelength bands. A combiner combines the modulated first, second, and third wavelength bands. A diverter diverts a portion of the combined modulated wavelength bands to a sensor. The sensor senses a relative position of each of the fiducials and sends the position information to a microprocessor. The microprocessor then determines an error based on the relative position of the fiducials. The microprocessor then sends a signal to at least one component of the system to resolve the error.
Consistently better image quality can be achieved by eliminating thermal errors in the mounting of image modulating devices in real time providing perfect registration over a wide temperature range. An additional benefit is that the initial alignment to mount the image modulating devices need only be as good as a few pixels simplify the fixuring and procedures required. The design of the projector can also be made simpler by not requiring as good thermal stability of the image modulating devices and associated optics mounting.
The border pixels of the three separate image modulating devices to provide a fiducial pattern that can be used for alignment, for example multiple pixels in each corner of the device. Upon recombination of the light from each of the three devices, the single light path is split into two components. The majority of light will be the normally projected image area, while the remainder of light, containing the fiducial information in the border areas is cropped. This small amount of light (also containing the fiducial surround) is directed to a sensor, which will be used to determine the pixel locations of the fiducial pattern and in turn, determine the required positional. Determination of which device requires positional adjustment can be done utilizing many means, including, but not limited to a selective application of filters to separate the colors, or by illuminating the fiducial patters on the three image modulating devices in a predetermined sequence. If a correction is required, actuators on the red and blue image modulating device mounts are powered to bring them back into alignment with the green reference channel.
In one embodiment of this idea, if alignment to within a half pixel is adequate, it would be possible eliminate the actuators and make all of the correction electronically, shifting where the row and column data begins and ends separately for the three channels. The simplest useful correction would be a simple x and y translation which would require only one fiducial point. A more useful correction accounting for possible skew requires at least two fiducial points. Embodiments performing these types of correction can be implemented moving optical components such as mirrors, prisms or simply the image modulating devices.